java - 如何避免从外部世界创建 B 类对象，并只允许通过 A 类

Question

如何避免从外部世界创建 B 类对象，而只允许通过 A 类？

我有两个类

public class A {
    B obj = null;
    public A() {
        obj = new B();
    }        
    public void methodA () {
        obj.methodB();
    }        
    // other methods
}

public class B {
    public void methodB () {
        // some logic            
    }
    //other methods
}

public class Client {
    public static void main (String s[]) {
        // Valid Call            
        A obj = new A();
        obj.methodA();  // Since methodB is called internally

        // Invalid Call , How to restrict this B object creation here ?
        B objB = new B();
        objB.methodB();

    }
}

Answer 1

我能想到的一个解决方案是使用 A 的内部静态类:KeyToB 和私有(private)构造函数，B 需要实例化该类。因此，只有 A 可以实例化 B，B 可以位于不同的文件中。

public class A {
    B obj = null;
    public A() {
        obj = new B(instance);
    }
    public void methodA() {
        obj.methodB();
    }
    // other methods ..

    //The key to have the right to instanciate B, only visible in A
    private static KeyToB instance = new KeyToB();

    public static class KeyToB{
        private KeyToB() {
        }
    }
}

public class B {
    //The constructor is package visible, it need a non null instance of KeyToB . If someone use new B(null), it will get a RuntimeException
    B(KeyToB instance) {
        if (instance == null){
            throw new IllegalArgumentException("B can only be instanciated by A");
        }
    }
    public void methodB() {     
    }   
}